1. Technical Field
The present invention is directed to radio-frequency (RF) amplifiers, in particular to RF amplifiers in RF mixers.
2. Background Information
RF amplifiers may be used for pre-amplifying RF signals received from an antenna and/or a subsequent low-noise amplifier (LNA) before providing the amplified signals to a mixer, such as a Gilbert mixer shown in FIG. 7. The Gilbert mixer comprises a pre-amplifying stage 701 and a mixing circuit 703 mixing the amplified signals provided by the pre-amplifying stage 701 using local oscillator signals in order to obtain e.g. base band or intermediate frequency (IF) signals. The Gilbert mixer shown in FIG. 7 may comprise MOS switches steering the signals and the DC to the load. However, the switches generate 1/f noise which appears at the output of the mixer during switch-transitions.
The amount of the 1/f noise depends on the applied technology. For example, the 1/f noise increases with decreasing sizes associated with current chip technologies. To reduce the 1/f noise, a passive mixer as shown in FIG. 6 may be used. The passive mixer comprises a pre-amplifying stage 601 for pre-amplifying RF signals from e.g. an antenna and/or a balloon and may be designed to form a low-noise amplifier (LNA). The amplified RF signals are provided to a mixing circuit 603 comprising a small capacitor 605 for removing high-frequency components. The mixing circuit 603 comprises an operational amplifier 607 forming, for example, a first intermediate frequency amplifier with a low-pass response and comprising single pole low-pass filters (LPF) with a capacitor and a resistor, respectively. The output signals of the operational amplifier 607 may be provided to an intermediate frequency filter. The 1/f noise is reduced, since the bias current (DC) of the preceding gain stage 601 does not flow through the switches and therefore does not generate the 1/f noise. The bias current in the LNA 601 is defined by the thermal noise. The higher the current in this gain stage, the better is the noise figure of the mixer. However, with increasing current in the gain stage the power dissipation in the mixer increases.